1. Field of the Invention:
This invention relates to the production of multilayered printed-wiring-boards, and more particularly to a method of manufacturing a multilayered printed-wiring-board by electrically connecting a plurality of printed-wiring-boards (hereinafter called "PWB") together.
2. Description of the Related Art:
Heretofore, attempts have been made to install circuit elements on a substrate with high-concentration to realize an integrated circuit board. To this end, it is a known practice to use a multilayered PWB, in which insulating and conductive layers are placed alternately one over another. Another popular method is to use a multilayered printed-circuitboard (hereinafter called "multilayered PCB"), in which various circuit elements are installed on and/or inside a multilayered PWB.
The multilayered PWB and PCB are particularly useful when realizing a compact-sized and high-concentration circuit of high frequency, such as 400
MHz band, 800-900 MHz band and 1.5 GHz band, in automobile and portable telephones, for example.
In the manufacture of a multilayered PWB or PCB, it is known to unified a plurality of PWBs by superimposing.
As is well known in the art, a PWB is produced by providing a conductive layer on the surface of an insulating substrate and forming an additional conductive layer thereover in a desired pattern according to need. A copper foil or a conductive paste, for example, is used for the conductive layer.
Further, a PWB having a conductive layer on only one of opposite surfaces is called a single-face PWB, while a PWB having a conductive layer on either one of opposite surfaces is called a double-face PWB.
FIGS. 13 through -5 are cross-sectional views showing progressive manufacturing steps of a multilayered PWB according to the typical conventional method, which will now be described in greater detail. The conventional production of a multilayered PCB is similar to that of a multilayered PWB except for an additional step of installing circuit elements, and therefore, its detailed description is omitted here for clarity.
In FIG. 13, two single-face PWB 10, 12, a double-face PWB 14, and two inter-layer adhesive sheets 16, 18 are shown.
Each of the single-face PWB 10, 12 is composed of an insulating substrate 20, 22, and a conductive layer 24, 25 in the form of a copper foil attached on one surface of the insulating substrate 20, 22. The conductive layer 24, 25 is free of patterning.
The double-face PWB 14 is composed of an insulating substrate 26, and a conductive layer 28, 30 attached on either one of opposite surface of the insulating substrate 26. The conductive layer 28, 30 is previously formed in a pattern necessary to realize a desired circuit.
The conductive layer 24, 25 has a thickness of about 18 to 35 .mu.m, for example, while the conductive layer 28, 30 has a thickness of about 70 .mu.m, for example. Thus the conductive layer 28, 30 is larger in thickness than the conductive layer 24, 25 for securing reliability of connection between the conductive layers 28, 30 and the through-holes after the through-hole forming described below.
The inter-layer adhesive sheet 16, 18 is a sheet commonly called "prepreg". The inter-layer adhesive sheet 16, 18 is formed of an epoxy resin material curable under a predetermined curing condition (i.e., temperature and time). In general, this material is very popular for inter-layer adhesive sheets.
In production, first of all, these components are placed one over another.
For example, as shown in FIG. 13, the single-face PWB 10, the inter-layer adhesive sheet 16, the double-face PWB 14, the inter-layer adhesive sheet 18, and the single-face PWB 12 are placed one over another in this order.
Further, the conductive layer 24 of the single-face PWB 10 faces upwardly, while the conductive layer 25 of the other single-face PWB 12 faces downwardly. Namely, these two single-face PWBs 10, 12 are placed in such a manner that their respective conductive layers 24, 25 face outwardly in opposite directions.
In this superimposed position, all of the superimposed components are heated at a predetermined temperature for a predetermined time, with applying a pressure in the direction of superimposing. At that time the adjacent superimposed parts come into contact with each other.
For example, all of the superimposed components are clamped between a pair of flat stainless plates in the direction of superimposing, and are pressed by a pressure of 30 kgf/cm.sup.2 At that time, they are heated at a temperature higher than a curing temperature of the inter-layer adhesive sheet 16, 18, e.g., 170.degree. C., for 90 minutes.
Under this pressure and heat, the inter-layer adhesive sheets 16, 18 are cured to form a pair of insulating layers 32, 34 covering the conductive layers 28, 30 of the double-face PWB 14. In addition, the single-face PWBs 10, 12 are adhered one to each surface of the double-face PWB 14 by the respective insulating layers 32, 34.
Then, sub-through-holes 36 are formed through the superimposed components at predetermined positions in the direction of superimposing, as shown in FIG. 14.
As the inner surface of the individual subthrough-hole 36 is plated to extract copper, the conductive layers 24, 28, 30, 25 are electrically connected at portions where the sub-through-holes 36 are formed, as shown in FIG. 15. That is, the conductive layers 24, 28, 30, 25 are electrically connected by through-holes 38 having an inner surface coated with copper.
Subsequently, the conductive layers 24, 25 are processed by patterning. If the patterning was done before forming the sub-through-holes 36, an accurate pattern could not have been achieved. This is the reason why the patterning is done after having formed the through-holes. As a result, a multilayered PWB has been completed in which a desired circuit is formed as patterns of the conductive layers 24, 28, 30, 25. In this conventional case, there are four conductive layers, which thus constitute a four-layered PWB.
Thus in the conventional art, a plurality of PWBs are superimposed into a unitary form, and then the conductive layers of the PWBs are electrically connected by through-holes.
However, if the sub-through-holes are formed after the PWBs have been superimposed into a unitary form, good smearing is difficult to achieve.
Further, in the foregoing manufacturing method, after having formed the sub-through-holes, it is necessary to take a wet processing such as plating and patterning.
But if a liquid penetrates into the multilayered PWB during the wet processing, it would result in various troubles such as oxidation of the conductive layers, oxidation of the conductive layers and shortcircuit between the patterns.
With the foregoing conventional methods, only a limited rate of production of good quality multilayered PWBs can be achieved. The same can be said of multilayered PCBs.